Electrically powered artificial lighting for general illumination purposes has become ubiquitous in modern society. Electrical lighting equipment is commonly deployed, for example, in homes, buildings of commercial and other enterprise establishments, as well as in various outdoor settings. The light sources utilized in luminaires for general illumination have evolved from traditional sources, such as incandescent or fluorescent lamps, to increasingly efficient solid state light sources. The most common form of solid state light sources utilized in luminaires is the light emitting diode or “LED.”
LED based general illumination lighting, however, has limitations. LEDs, for example, typically emit light over a rather broad angular output field, typically called Lambertian angular distribution with 120-degree beam angle (full-width at half-maximum). Even with optical elements to somewhat narrow the output angle range, some light often is lost outside the desired area of illumination. To achieve desired overall lumen output, luminaires for most general lighting applications have some number of LEDs. Due to the wide angular distribution, the LEDs usually are deployed in an array or other grid pattern of point sources.
Laser light sources are good pumping sources and have high power in a relatively small package with extremely strong directionality. A phosphor or other photo luminescent material pumped by ultraviolet (UV) or blue light from a laser emitter produces longer wavelength light. With an appropriate phosphor, for example, such laser light may be converted into a white light output. Due to safety concerns and low optical efficiency, however, laser light sources are typically not utilized as a light source for general illumination in the lighting industry. If not fully converted or otherwise filtered out, UV may be harmful to the skin or eyes of people exposed to illumination from a luminaire that uses UV pumped phosphor. Blue laser light is not dangerous because of the wavelength of blue colored light, but instead may be harmful because the laser light beam is highly focused and coherent, resulting in a high power density light source.
Although blue laser light sources have been utilized in automobile headlamp applications, the designs for those lighting devices involve several mirrors to deflect the blue laser light and have many air gaps. The air gaps and mirrors in the design of such lighting devices may be problematic for several reasons. In the event of breakage of the lighting device (e.g., during an automobile accident), laser light containment may be compromised so as to potentially allow the blue laser light to escape outside, which can harm a living organism exposed to the blue laser light directly, or even indirectly. Accordingly, incorporating a blue laser light source into a luminaire for general illumination in a safe and optically efficient design is difficult.
Instead, most general illumination lighting therefore utilizes a group of series connected white LEDs of approximately the same brightness capacity mounted on a printed circuit board to form an LED based light engine. The LEDs are mounted on a printed circuit board, and assembly of a luminaire requires mounting of one or more secondary optics to process the light from the LEDs to produce a desired light output distribution. This approach, however, limits the types of light output distributions that can be produced by LED based luminaires, particularly without requiring complex and/or costly LED arrangements and circuit boards. For example, LED based luminaires utilize rigid printed circuit boards. Because of the large number of LEDs and attendant need for a larger circuit board, LED light engines are difficult to adapt to curved or irregular luminaire configurations.
If color tuning is desired, the light engine may include two or more groups of LEDs of different color characteristics, e.g. white (W) LEDs of two different color temperatures, three of more strings of different color LEDs (such as red (R), green (G) and blue (B) or combinations of white and colors, e.g. RGBW). The inclusion of multiple groups of different LEDs increases the number of LEDs on the circuit board, which increases the complexity of the layout of elements a connection traces on the board. The inclusion of multiple groups of different LEDs also increases the complexity of the control circuitry, for example to provide multiple channels of control for the different groups/types of LEDs.
As noted, LED based luminaires often include secondary optics to direct the light from the LEDs to provide a light output distribution suitable for the intended general illumination application of the luminaire. Most such luminaires are not tunable with respect to output distribution. Instead, luminaires intended for different applications, for example for a wall washing application as opposed to a downlight application, typically have different static secondary optics.
Dynamic variation of the light output distribution adds a still further degree of complexity and attendant cost. For example, one approach uses controlled variable secondary optics, which increases cost of the optic and requires additional control circuitry. Another approach utilizes multiple LEDs coupled through a complex passive lens, with different distributions based on operations of different ones of the LEDs thought different portions of the lens. The additional LEDs increase the complexity of the printed circuit board layout and require additional control channels from the driver circuitry.
As the number of LEDs increases, for tuning of color characteristic or tuning of output distribution, it becomes difficult to keep the luminaire compact, due to the size and number of the LEDs. As noted, the complexity of the printed board layout increases, and the requirement of more control channels increases the cost of the driver circuitry. Assembly time and cost also increase. The increased number of LEDs also raises thermal issues relating to dissipation of increased heat generated by more LEDs.
There is room for improvement in solid state lighting for general illumination to address some or all of the issues outlined above.